Alumina refractories

ABSTRACT

Disclosed herein is a refractory brick batch mix which consists essentially of, by weight, about 85 to 95 percent alumina, about 2.99 to 12.99 percent silica, about 0.01 to 0.5 percent of a lithium compound capable of oxidizing to lithium oxide, and about 2 to 7 percent volatilized silica. Refractories prepared from these mixtures exhibit increased strength, higher density, lower porosity and higher refractoriness compared to those previously available.

United States Patent Bakker 1 Mar. 28, 1972 [s41 ALUMINA REFRACTORIES 3,067,050 12/1962 Miller ..l06/65 [72] Inventor: Wale T. Bakker, Severna Park, Md. Primary Examiner james E e [73] Assignee: General Refractories Company, Philadel- A'mmeyfEVere" yJ and Francis 11 Neruda h' P p a 57 ABSTRACT F'l d: l 7 1969 [22] l e y Disclosed herein is a refractory brick batch mix which consists [21] App1.No.: 839,663 essentially of, by weight, about 85 to 95 percent alumina, about 2.99 to 12.99 percent silica, about 0.01 to 0.5 percent of a lithium compound capable of oxidizing to lithium oxide,

[52] U.S. Cl ..106/65, 106/67, 106/68 and about 2 to 7 percent volatilized Silica Refractories [51] Int. Cl. v ..C04b 35/10 prepared from these mixtures exhibit increased strength 0f Search density lower porosity and ef actofiness compared to those previously available.

[56] References Cited 8 Claims, No Drawings UNITED STATES PATENTS A 2,559,343 7/1951 Caton "106 /65 ALUMINA REFRACTORIES This invention relates to refractories of high alumina content by which is meant, for the purposes of this invention, refractories containing at least about 50 percent by weight of A1 High alumina refractories are generally classified by their Al O content in groups having, approximately, 50, 60, 70, 80, 90, or 99 percent Al O by analysis. Those containing 50 to 90 percent of A1 0 are prepared by blending various high alumina refractory materials, while those containing as much as 99 percent A1 0 are essentially high purity alumina. The most common high alumina refractory materials and their typical A1 0; contents are fused alumina, 99.5 percent; sintered alumina, 99.5 percent; calcined alumina, 99 percent; fused bauxite, 95 percent; calcined South American bauxite, 88 percent; calcined Alabama bauxite, 74 percent; calcined diaspore, 76 percent, burley diaspore, 48 and 58 percent; and kyanite, 56 percent. All of these materials are chemically compatible, and so they can be blended to provide almost any desired resultant alumina content.

The present invention is concerned with those high purity high alumina refractories containing by weight in the range of about 85 to 95 percent alumina. In addition to the alumina, these refractories generally contain in the range of about 5 to percent silica with minor amounts of impurities such as TiO Fe O alkalies and alkaline earths normally associated with the alumina and silica ores.

Ordinarily, high alumina refractory bricks or shapes are manufactured from relatively pure, high density A1 0 grog material and a siliceous binder. Traditional forms .of silica used for this purpose are various kinds of clays, usually kaolin or ball clay, and ground silica sand. The use of clay, at least in large amounts, is objectionable primarily because of its tendency to decompose during firing into mullite and a glassy phase, the glassy phase forming the bond between the grog grain, which structure decreases the refractoriness of the brick under load. The use of ground silica, on the other hand, increases the refractoriness under load of the brick, probably because the silica reacts with the alumina in the solid state to form mullite so that little or no glass is formed. However, because of the low reactivity of the silica, this reaction is slow and only very little mullite is formed at conventional burning temperatures. Therefore, these refractories have a low cold strength and most of the mullite in the brick is formed during service leading to an undesirable large reheat expansion.

It was discovered that high alumina refractories possessing high cold strength and low reheat expansion could be prepared by incorporating in the batch mix about 0.05 to 0.2 percent of lithium fluoride or lithium carbonate and about 0.01 to 0.3 percent of at least one compound selected from the group consisting of calcium hydroxide, magnesium hydroxide and iron oxide. Such refractories are described in US. Ser. No. 741,333 which was filed on July 1, 1968 and is in the hands of a common assignee and incorporated herein by reference thereto. While refractories prepared in this manner exhibit characteristics superior to those of the prior art, they lack sufficient high pressed density and low porosity. Accordingly, the discovery of a means whereby these shortcomings could be overcome simply and economically would constitute a significant and important advance in the art. The present invention provides just such a means.

It is among the objects of this invention to provide high alumina refractories which, as compared with those previously available, are of increased strength, higher density, lower porosity, and higher refractoriness, and which may be produced from readily available high alumina materials by practices which are customary in the refractory trade.

Another object is to provide a brick batch mix suitable for producing refractories in accordance with the foregoing object.

A further object is to provide a simple and economical method of making refractories in accordance with the firstnamed object.

Still other objects and advantages will become apparent from a consideration of the following specification and claims.

toriness under load. In fact, improvement in the refractoriness under load has been realized through the stated additions.

More specifically, the present invention involves a high purity alumina refractory brick batch mix consisting essentially of, by weight, about to percent alumina; about 2.99 to 12.99 percent silica; about 0.01 to 0.5 percent of at least one lithium compound; and about 2 to 7 percent of volatilized silica. Preferably, the refractory brick batch mix will consist essentially of, by weight, about 88 to 92 percent alumina; about 4.95 to 8.95 percent silica; about 0.05 to 0.2 percent lithium compound; and about 3 to 5 percent volatilized silica. The percentages employed herein are based on the total weight of the refractory mix.

As noted hereinbefore, alumina is available in varying grades and purities. Generally the alumina employed in this invention will be a high purity" grade, that is, it will contain at least about 99 percent A1 0 preferably at least about 99.5 percent A1 0 The alumina grog may comprise alumina which is sintered, tabular, fused, calcined or the like. However, as is common in high alumina refractory brick manufacture, a significant portion of the alumina will ordinarily constitute relatively coarse particles, and the remaining portion will be made up of relatively fine particles. Thus, for example, from about 50 to about 65 percent, by weight, of the alumina will be in the range of 6 to +48 mesh (Tyler) and the balance (from about 35 to about 50 percent, by weight) will be 48 to 400 mesh. Preferably, at least about 10 to 30 percent by weight, of the alumina will be 325 mesh or finer.

Similarly, the silica component will also be a high purity material, that is, containing at least about 99 percent SiO,, preferably over 99.5 percent Si0 and especially about 99.9 percent SiO In this respect, ground glass sand is particularly suitable. Generally, silica will be substantially 200 mesh, and preferably at least about 50 percent, by weight, thereof will be 325 mesh or finer.

In addition to the alumina and silica, the refractory brick batch mix will-have incorporated therein volatilized silica and a lithium compound. The lithium compound employed should be capable of forming, that is, oxidizing to lithium oxide during the firing of the pressed brick batch mix when said lithium compound is present therein. Lithium carbonate and lithium fluoride are particularly suitable in this respect. Lithium fluoride is especially preferred. However, the lithium carbonate is quite desirable because of its relative cheapness and ease of handling. Whichever lithium compound is employed, such should be relatively finely divided, that is, substantially all thereof should be l00 mesh, and preferably 325 mesh. As previously indicated, the amount of lithium compound incorporated in the mix should constitute about 0.01 to 0.5 percent thereof. Higher amounts markedly decrease the refractoriness under load of the resulting brick.

volatilized silica, as noted, is the other essential component incorporated in the brick batch mix. It is an extremely fine grain almost pure SiO Of course, like most substances derived from naturally occurring materials, the composition thereof will generally vary slightly. However, in this instance, that is, commercially available volatilized silica, the variation will be very minor.

By way of further elucidation, a typical volatilized silica would have the following chemical analysis:

Component Percent SiO, 95.9 M 0, 0.6 TiO, 0.02 mo, 0.4 CaO 0. l MgO 0.4 Alkalis 0.4

Loss on lgnition 2.0

Depending upon the other components of the brick batch mix, varying amounts of volatilized silica will be necessitated in order to achieve the benefits of this invention. The amount of volatilized silica utilized will be in the range, by weight, from about 2 to 7 percent and preferably from about 3 to percent. It is, of course, well within the skill of one knowledgeable in the art to determine the most satisfactory combination of components within the stated ranges for any particular brick batch mix.

As stated, the refractory brick batch mix of the present invention consists essentially of alumina, silica, a lithium compound which will form lithium oxide during burning, and

volatilized silica. However, small amounts of other materials may be incorporated in the batch mix without deleteriously affecting the properties of the resulting brick. For example, phosphoric acid may be included to enhance the cold strength. Other binders, such as sodium lignosulfonate may be included to impart green strength and lubrication. When phosphoric acid is employed, it will usually be present in the range of about 1 to 4 percent, by weight, preferably from about 2 to 3 percent (as a 75 percent aqueous solution of H PO,). On the other hand, when a lignosulfonate binder is employed, it will ordinarily be present in an amount of from about 1 to 2 percent, by weight, as a 50 percent solution in water or from about 0.5 to 1 percent, on a dry basis.

Generally, in preparing bricks from the mixes described herein, the mix will first be tempered with a small amount of water. Some or all of the water may be provided by binders of the type discussed above. In general, the total water will range from about 2.5 to 6 percent, by weight, preferably from about 3 to 5 percent.

In preparing the mix, the materials may be blended in accordance with conventional refractory practice. For example, a muller mixer may be employed in which case it is preferred to first add the coarse materials to the pan along with the major portion of the moisture. This is followed by the finer materials and the remainder of the moisture. Mixing is continued until proper consistency is achieved.

The tempered mix is then pressed into the desired refractory brick shape. It should be noted that the term brick is used in its generally accepted meaning in the refractory art. Specifically, such is not intended to limit in any way the design or physical configuration of the refractories that can be prepared in accordance with this invention. Generally, pressing of the brick necessitates a pressure of at least about 4,000 p.s.i. which may range up to about 15,000 p.s.i. A preferred pressure is in the range of about 8,000 to about 10,000 p.s.i. After pressing and drying, the shaped brick is fired at a temperature which is effective to provide the ceramic bond. Ordinarily, firing is conducted at a temperature which may range from about l,200 to l,700 C. and preferably l,300 to 1,500 C.

The many facets of this invention are further illustrated by the following examples which are not to be construed as limitations thereof. On the contrary, resort may be had to vari ous other embodiments, modifications and equivalents of these examples which readily suggest themselves to those skilled in the art without departing from the spirit of the present invention and/or the scope-of the appended claims.

EXAMPLE I A brick batch mix is prepared from the following: 60 percent, by weight, 6 mesh sintered alumina coarses; 20 percent -48 mesh sintered alumina fines; 10 percent 325 mesh calcined alumina; 6.5 percent 200 mesh silica sand and 3.5 percent volatilized silica. Thereupon, lithium fluoride is incorporated into a separate portion of the mixture, so as to constitute 0.05 percent by weight thereof. For identification purposes, the mixture without lithium fluoride will be referred to as Mix A and themixture containing such as Mix B.

In a similar manner, another brick batch mix is prepared without having any volatilized silica therein. The composition of this mixture (Mix C) is as follows: 50 percent, by weight, 6 mesh sintered alumina coarses; 25 percent 48 mesh sintered alumina fines; 15 percent 325 mesh calcined alumina and 10 percent 200 mesh silica sand. Each of these mixes is then tempered with an aqueous magnesium lignosulfonate solution and hydraulically pressed into 9X4%X2% inch bricks 'at 10,000 p.s.i. Thereafter, the bricks are fired for 5 hours at 1,450 C. After cooling, the physical properties and characteristics of the fired specimens are determined by means well known to those skilled in the art. The compositions of each of the three brick batch mixes along with the physical properties of the bricks produced therefrom are set out in Table 1.

It is immediately apparent from the above Table that the bricks prepared from Mix B are markedly less porous when compared'to those of Mix A and Mix C. Likewise, the Mix B bricks evidenced substantially greater strength than the other bricks. For example, at room temperature the Mix B bricks had amodulus of rupture of 5,400 p.s.i. whereas the Mix A and Mix C bricks were only 2,600 and 2,730 respectively.

Thus, it is evident that unexpected superior results are obtained when a brick batch mix is prepared in accordance with the teachings herein.

I claim:

1. A refractory brick batch mix consisting essentially of, by weight, about to percent alumina, about 2.99 to 12.99 percent silica, about 0.01 to 0.5 percent of at least one lithium compound and about 2 to 7 percent volatilized silica, said lithium compound being capable of forming lithium oxide during the firing of the pressed brick batch mix.

2. A refractory brick batch mix according to claim 1 consisting essentially of, by weight, about 88 to 92 percent alumina, about 4.95 to 8.95 percent silica, about 0.05 to 0.2 pervcent of said lithium compound and about 3 to 5 percent volatilized silica.

3. A refractory brick batch mix according to claim 1 wherein said alumina contains at least about 99 percent Al,0; and said silica contains at least about 99 percent SiO,.

4. A refractory brick batch mix according to claim 3 wherein said alumina is at least about 99.5 percent A1 0, and said silica is at least about 99.9 percent SiO,.

5. A refractory brick batch mix according to claim 3 wherein at least about 10 to 35 percent, by weight, of said alumina will be 325 mesh or finer and at least about 50 percent, by weight, of said silica will be -325 mesh or finer.

6. A refractory brick batch mix according to claim 1 wherein said lithium compound is selected from the group consisting of lithium fluoride and lithium carbonate.

7. A refractory brick batch mix according to claim 6 wherein said lithium compound is lithium fluoride.

8. A refractory brick batch mix according to claim 1 wherein there is incorporated a binder selected from the group consisting of phosphoric acid and sodium lignosulfonate. 

2. A refractory brick batch mix according to claim 1 consisting essentially of, by weight, about 88 to 92 percent alumina, about 4.95 to 8.95 percent silica, about 0.05 to 0.2 percent of said lithium compound and about 3 to 5 percent volatilized silica.
 3. A refractory brick batch mix according to claim 1 wherein said alumina contains at least about 99 percent Al2O3 and said silica contains at least about 99 percent SiO2.
 4. A refractory brick batch mix according to claim 3 wherein said alumina is at least about 99.5 percent Al2O3 and said silica is at least about 99.9 percent SiO2.
 5. A refractory brick batch mix according to claim 3 wherein at least about 10 to 35 percent, by weight, of said alumina will be -325 mesh or finer and at least about 50 percent, by weight, of said silica will be -325 mesh or finer.
 6. A refractory brick batch mix according to claim 1 wherein said lithium compound is selected from the group consisting of lithium fluoride and lithium carbonate.
 7. A refractory brick batch mix according to claim 6 wherein said lithium compound is lithium fluoride.
 8. A refractory brick batch mix according to claim 1 wherein there is incorporated a binder selected from the group consisting of phosphoric acid and sodium lignosulfonate. 